The present invention concerns a novel process for the production of communication-transmitting optical fibers.
Qualified communication fibers must be distinguished by low transmission losses of less than 10 db/km., preferably less than 6 db/km., and by a low pulse dispersion of less than 4 ns/km., preferably less than 2 ns/km., if they are to be suitable for the transmission of information over more than 1,000 m. with an adequate bandwidth of several hundred megahertz. Such values are only achieved by fibers, the refractive index profiles and geometries of which have been accurately predetermined, and heretofore could be attained only to an inadequate extent.
As is known, two CVD (Chemical Vapor Deposition) methods are employed for the production of preforms for communication fibers.
One method, the external cladding technique, is based on the deposition of glass soot by means of a pyrolysis burner on a quartz glass rod. (See, e.g., U.S. Pat Nos. 3,711,262 or 3,823,995). After the cladding step, the rod can be pulled out and the hollow cylinder is sintered to produce the preform and then drawn into a fiber.
In the other method, the internal cladding technique, a quartz tube is heated to such an extent that metal halides, e.g., of Si, Ge, P, B, Sb, etc. and oxygen, with which compounds the carrier gas stream flowing therethrough is laden, are decomposed by pyrolysis and are deposited on the inner wall of the tube; the carrier gas stream can likewise be O.sub.2 in the process. (For example, see DAS No. 2,546,162; British Patent Application No. 42779/76; and U.S. Pat. application Ser. No. 732,197, and U.S. Pat. application Ser. No. 941,691, filed on Sept. 12, 1978, whose disclosures are incorporated by reference herein).
Cladding takes place generally in two phases. First of all, optical insulating layers are applied, primarily B.sub.2 O.sub.3 -SiO.sub.2 systems (e.g., British Pat. No. 1,426,605; J. Am. Ceramic Soc. 58 (5-6), 261, (1975) and U.S. Pat. No. 3,963,468, as well as copending U.S. application Ser. No. 941,691 filed on Sept. 12, 1978.
Thereafter, the core layers are produced wherein the index of refraction rises from the first core layer to the last core layer, generally parabolically (see above cited references). In this process, the next following step is particularly critical, wherein a solid rod (the preform) is produced from the thus-coated tube by collapsing. At this point, irregularities and flaws are produced time and again in the originally circular cross section of the tube. This deviation from the circular shape is also entrained into the fiber, leading to an impairment in geometry and thus to poorer physical characteristics, especially the pulse dispersion.
Solutions for this problem have been suggested, DOS [German Unexamined Laid-Open Application] No. 2,711,295 describes a process wherein the collapsing step is executed under positive counterpressure. However, this process cannot be integrated into the cladding procedure, but rather demands an interruption of the process and is complicated. As a consequence, damaging effects (water, dirt particles) can negatively affect the properties of the layers and of the subsequently produced preform.